Chemical looping is a recently developed process which can be utilized in electrical power generation plants which burn fuels such as coal, oil, natural gas, biomass, and other fuels. The chemical looping process can be implemented in existing or new power plants, and provides promising improvements in terms of reduced plant size, reduced emissions, and increased plant operational efficiency, among other benefits.
The FIG. 1 depicts a chemical looping system 10 that comprises an oxidizer 12 and a reducer 14. In the oxidizer 12, a solid such as calcium sulfide (CaS) or a multivalent metal oxide (denoted as “Me”) are oxidized with oxygen derived from air. For example, calcium sulfide is oxidized in the oxidizer 12 to calcium sulfate. The multivalent metal oxide is oxidized from a lower valence state to a higher valence metal oxide (e.g., FeO is oxidized to Fe2O3). The calcium sulfate and the metal oxide are also called solid oxygen generators. Nitrogen and oxygen are released from the oxidizer as hot exhaust gases. The hot calcium sulfate is then transported to a reducer 14, where calcium sulfate is reduced to calcium sulfide with the release of oxygen. The released oxygen is used to combust a fuel (e.g., a solid fuel such as coal) supplied to the reducer 14. The combustion of the solid fuel in the reducer 14 produces carbon dioxide and small amounts of water along with other contaminants (hereinafter called “flue gases”). The reduced calcium sulfide from the reducer is then discharged back to the oxidizer 12 thus providing the chemical loop and the thermal loop for the process.
In summary, a chemical looping system utilizes a high temperature process, whereby solids such as calcium- or metal-based compounds are “looped” between a first reactor, called an oxidizer (or an air reactor), and a second reactor, called a reducer (or a fuel reactor). In the oxidizer, oxygen from air injected into the oxidizer is captured by the solids in an oxidation reaction. The heat liberated by the oxidation raises the temperature of the solids and the gases. The captured oxygen is then carried by the hot oxidized solids to the reducer to be used for combustion and/or gasification of a fuel such as coal, for example. After a reduction reaction in the reducer, the solids, no longer having the captured oxygen, are returned to the oxidizer to be oxidized again. This cycle is repeated.
Even though gases are generated by combustion in the reducer and in the oxidizer, there is a need to use a lift gas in both the oxidizer and the reducer in order to facilitate lifting the solid fuel and the solid oxygen generator (CaSO4 or the metal oxide) into a combustion zone in the oxidizer and reducer so as to facilitate efficient combustion. In order to do so, an external lift gas is fed to either the oxidizer, the reducer, or to both. In the case of the oxidizer, the air provides the lift gas as well as the source of oxygen. In the case of the reducer, a gas that is reactive with the fuel is desired for providing lift. This gas can either be steam from the steam generation part of the power plant or carbon dioxide from a gas purification unit (GPU). Since the final product from the combination of the reducer and the gas processing unit in sequence is a relatively pure carbon dioxide stream, it is therefore desirable to use a lift gas that does not add additional contaminants to the flue gases.